I just have an old Dlink DIR-615 Wireless router. I have some problems with it and need to improve it... So, what is the best option... buy a new DIR-636L or 657 router or install the latest ddwrt firmware on the existing one?

Depends what precisely your problems are? When I was with Virgin I installed DD-WRT on my 615 and it performed perfectly after that, but if you're looking for better wifi performance, DD-WRT probably isn't going to do that. However the 2 routers you have selected as alternatives are also single band so whether you'd get much improvement from them is debatable

The problem is (as I posted in here "http://superuser.com...a-wimax-adsl-co"), I have a Wimax ADSL connection. The router connects to the net through a modem (IDU). I just read about dual band routers. It seems to be they are useful when the router is connecting to the net through internet. In my home, only about 1 to 2 devices connect through Wifi. Other devices use cables. So my question is, will it be pointless to have a dual band router in my setup?

I did have some Wifi range issues which were fixed by the ddwrt firmware. Also had internet disconnection issues which pointed to a faulty modem which was replaced by the ISP. But recently, I read some reviews on this router (http://reviews.cnet....7-32435592.html) and they are not good at all. So, can the DDWRT firmware fix all these issues?